Pacus are primarily vegitarian/fruit eaters in nature, but like many fish, they are opportunists, and will occasionally consume some protien.
My advice, is to get some fresh spinach from your grocer, wash it up real good, and then nuke it in the microwave for a few seconds, just enough to make it wilt... then put it in the tank and watch your pacu's devour it... it's a good source of fiber and nutrients... you might want to incorporate it into their diet on a weekly basis... I have with my wifes orandas (her fish, I take care of them) and they love it... I give them red leaf and romaine lettuce as well...
avoid iceberg, nutritionally, iceberg is WORTHLESS.
I don't know if you are aware, but PACU's are just like, a 1/2 step removed from piranah, and sometimes when they are wild caught, you will end up with a piranah instead of a pacu. This happened to me when I was a kid... I was able to tell by the teeth, and the missing chunks of flesh in my smaller fish.
I sold him back to the fish store, though, I hear if you keep them fat, they stay dumb and happy... in the wild, even the most feared piranah, the caribe, only stream to blood, and they are usually only a real problem during the dry season when they eat everything in site including themselves.